Highly specific antiserum to androgen binding protein, tubulin, myosin, actin, relaxin and clathrin has recently become available. During the past year we have succeeded in localizing these proteins to certain sites in the testis and epididymis using the peroxidase-antiperoxidase method of Sternberger on frozen sections. It is now logical to attempt to localize these proteins in Epon sections cut for electron microscopy. The EM immunohistochemical localization of antigens is fraught with numerous difficulties, but we expect to overcome many of the problems. This will be the thrust of our efforts during the next 12 months. We hope to learn more about the mechanism of secretion and absorption of ABP in the reproductive tract. Localization of structural proteins in Sertoli cells may provide new insights concerning the complex interactions of Sertoli and germ cells.